1. Technical Field
The present invention relates to a filter having a switch function and a band pass filter, and more particularly, to a filter having a switch function suitable for a radio frequency (RF) communication device used in common for an antenna in a base station for a cellular phone adopting time division duplex scheme.
2. Related Art
Conventionally, a RF communication device used in common for an antenna by time division duplex scheme realizes transmission of baseband signals by switching between a transmission circuit and a reception circuit through time division using the same frequency band. In this kind of RF communication device, an RF switch circuit 74 having a construction of single pole double throw (SPDT) is installed between transmission/reception circuits (TX circuit 71 and RX circuit 72) and an RF filter circuit 73 as illustrated in FIG. 24, to perform switching a transmission path. Also, the RF switch circuit 74, for example, is configured by mounting an active device such as a PIN diode onto a microstrip line.
In a conventional RF communication device, respective circuits such as the transmission circuit 71 and the reception circuit 72 are formed as single elements, and they are connected with each other using a coaxial cable and the like. However, since the number of electrical and mechanistic components increases in this case, device costs may easily increase, and also, a transmission line of RF signals is lengthened, which increases a transmission loss of the circuit.
Japanese patent application publication No. 2005-51656 proposes a filter having a switch function that integrates an RF filter circuit and an RF switch circuit by installing PIN diodes D1e and D2e between an ANT terminal and an RX terminal, and between the ANT terminal and a TX terminal, respectively, as illustrated in FIG. 25. Also, in FIG. 25, C1a to C6e designate capacitance components and TL1e to TL4e designate short-circuit line resonators.
This filter circuit is configured to switch a conduction state between the ANT terminal and the RX terminal, and between the ANT terminal and the TX terminal by controlling voltages applied to the PIN diodes D1e and D2e, and thus to realize a switch operation. According to the same circuit, the number of components can be reduced and simultaneously, the length of the transmission line can be shortened, so that device cost reduction or transmission loss reduction can be achieved.
However, since the filter circuit has a construction of mounting a circuit device such as a chip condenser and a resonator on a plane circuit, that is, a plate-shaped dielectric substrate, and connecting the circuit device on a microstrip line, the transmission loss of the filter may be increased by the dielectric loss of the dielectric substrate. An increase in the transmission loss of the filter causes an increase of power consumption in a transmission circuit of a wireless device, and also, is directly connected with deterioration of a noise figure (NF) in a reception circuit. In that case, use of a low-loss substrate can be considered, but such a substrate is expensive. Also, when a low-cost substrate is used, selectivity of a material is not sufficient, so that it is difficult to obtain desired characteristics.